"But it's all important" - assorted study advice for taking notes
Too often, I hear "take notes" and "highlight what's important" as study advice given to students. What they often neglect to discuss is how to determine what is important or useful and worth writing down, particularly during lectures where the material may be a lot less organized.
First of all, this is one of those skills that comes with practice. Over time, you'll be able to pick out what's key and relevant information. This is to say, in the beginning, you are going to take too many notes and focus on unimportant things, but that's helpful in the long term practice of picking out what's actually important.
I still take too many notes at first, but over time I condense them as I take more notes, extracting more and more of what is fundamental.
Based on my personal experience, here's some assorted advice on note-taking that works for me
Before you even attend the lecture:
If your professor provides a reading ahead of time related to the lecture, read it. Don't try to take notes on it or worry about retaining much. Just read it. Multiple passes over the same material is going to help you understand what keeps being repeated and what sticks out.
If you don't have time to do that reading, skip to the chapter summaries, skim, and/or read the titles and subtitles. Get an idea of how the information is structured.
If your professor provides the lecture notes ahead of time, read it. Again, don't worry about taking notes on it. Don't worry about studying it. Passing over it once is still helpful.
(Note that passively reading slides is mostly useful in the beginning, but it is by no means an effective study technique.)
If you come across a concept that is interesting to you, follow your curiosity and research it. Even if it's just a passing mention. Making multiple connections in your mind helps you understand the material, no matter how irrelevant.
If a professor's slides are good, they will have a hierarchical structure, multiple sections, or some other logical flow. This is important - they will form the main skeleton of your notes. Structuring information in some logical way helps you remember it.
So. What do you actually want to write down?
If possible, write directly on the slides and take notes on them, and don't write what's already on the slides...
unless it's a key equation, name, concept, or something otherwise worth memorizing. The repetition helps.
Highlight sparingly. Highlighted text should be no more than 20% of all of the text. (This isn't a hard rule, of course.)
Good professors want to make things easy on their students. Some of them might tell you to highlight or underline key concepts.
If your professor says something that makes perfect sense to you, even if it's the same thing written on the slide but worded differently, jot it down.
If your professor says something funny or interesting (to you) that is at least tangentially-related to the topic at hand, jot it down. I've had professors relate concepts to music lyrics, movies, television shows, memes, etc. Not all of it was funny, but I wrote down what I found funny.
If something your professor says reminds you of something else, jot it down.
Basically, anything that you can connect to the main material is definitely worth writing down, even if it will never show up on tests. Make as many neural connections as possible is the key to accessing that information quicker.
I used to use the Cornell outline method, and while I still recommend it, I think it's worth trying out multiple note-taking methods and templates and see what works for you. There are a lot of good ones, and there is no one optimal system that works for every single person. What matters is what works for YOU.
What don't you write down?
Don't write out text fully from the slides
Don't write out full sentences - sentence fragments and 1-2 words that are enough to jog your memory are enough
You are not a transcriber. Don't capture everything that is being said.
What do you do with your notes afterward?
After taking notes in lecture - and this is CRUCIAL - you have to review your notes. Try to do it on the same day and right after the lecture. Don't trust your future self to understand what your today self was talking about and elaborate on some of the vague notes.
Write out more notes afterward. Make study notes. Write your own summaries.
The key here is reducing pressure during actual lecture time. You know your notes are going to be reviewed after the lecture, so you can sacrifice some coherency. You know you're going to have better notes later, so you can avoid perfectionism. You know you're making multiple passes of the material, so write down what happens to stick during the lecture.
Look at other peoples' notes on the same topics. See how similar yours are. If you have trouble picking out what is critical, take notes on those topics you missed.
At the end of the day, your notes are a means to an end, which is learning the material. You don't need to have perfect notes.
What to do if your professor sucks, doesn't post anything ahead of time, their slides are terrible, etc.
Master the art of self-study. In lecture, focus on capturing the main concepts. Write anything that resonates or sounds important.
Later, research the topics. Consult the syllabus or title of the lecture on what the main topic at hand is. Then look up the topic on YouTube, Khan Academy, etc. and find someone else or some other resource who teaches the material that resonates with you more.
Sometimes it's a guessing game of what professors find important. However, at the end of the day, your professor isn't the sole source of information on the topic, and even if your grades in their class don't reflect it, you at least have an understanding of it.
Sometimes the most random trivia is going to be on tests and exams. It happens. It doesn't mean you had bad notes or bad study habits necessarily.